There are many human physiological characteristics which can be used to provide personnel identification for security purposes, such as fingerprint, retina, iris, DNA, or even facial features. Among the devices that are capable of distinguishing physiological characteristics of one person from another, fingerprint sensing technology has become widespread in use and is often used to provide secure access to sensitive electronic devices and/or data. Generally, capacitive fingerprint sensors may be used to determine an image of a fingerprint through measuring capacitance through each capacitive sensing element of a capacitive sensor. The higher the capacitance, the nearer the surface of an adjacent or overlying finger to the capacitive sensing element. Thus, fingerprint ridges provide a higher capacitance in an underlying capacitive sensing element than do fingerprint valleys.
Capacitive fingerprint sensors come in at least two varieties, namely active and passive. Active capacitive sensors are often used in electronic devices to provide biometric security and identification of users. Active capacitive sensors initially excite the epidermis of the sensed finger. Capacitance to the epidermis is measured at each capacitive sensing element. Specifically speaking, the capacitance may be measured or determined by measuring a capacitive sensing element's voltage and/or charge during a low voltage phase and a high voltage phase of a modulation frequency for the capacitive sensing element array. The difference in voltages may be used to determine capacitance.
Some prior arts teach to obtain the low voltage phase and the high voltage phase by use of a drive ring formed around the sensor which excites the sensed finger and the voltage and/or the charge at each capacitive sensing element may vary as the drive ring is modulated since the voltage potential of the finger changes with the modulation of drive ring.
However, the voltage applied to the drive ring may be limited. Commonly, the drive ring voltage is no more than 4 volts peak-to-peak. Voltages above this may be too high for exciting the finger; this excessive excitation may be detected by a person as a “tingling” or uncomfortable feeling in their finger. Although the exact voltage at which one can sense the tingling varies from person to person, a 4 volt peak-to-peak voltage is generally considered as the threshold beyond which the feeling is noticeable.
Since the drive ring's voltage is restricted to avoid user perception, the thickness of any dielectric overlaying the sensor may also be limited. The thicker the dielectric between sensor pad and finger, the more attenuated the resulting capacitance and the blurrier the fingerprint image becomes. For dielectrics having a thickness or more than approximately 100 microns, the fingerprint image may become unreliable.
Another limitation arises when other parts of the user's finger or hand or body may capacitively couple through earth ground to the system, or directly to the system ground when touching other parts of the system. This capacitive coupling from the user to the system may be highly variable depending on how the user is touching the device. This parasitic coupling attenuates the voltage that the drive ring is able to drive into the user's finger, and as such reduces the signal. The attenuation may be highly variable depending on how the user is touching the device.
Due to the aforementioned disadvantages of prior arts, a fingerprint sensing device that can acquire high-quality fingerprint images without depending on how the user is touching the device is desperately desired.